1. Field of the Invention
The present invention relates to a test system for measuring analyte concentration in a fluid sample. The invention also provides a biosensor for use in the system, notably a biosensor for measuring analyte concentration in biological fluids, for example glucose in whole blood.
2. Description of the Prior Art
Biosensors typically include an enzyme electrode comprising an enzyme layered on or mixed with an electrically conductive substrate, for example a non-mediated enzyme electrode such as described in US 2004/0061841. The electrodes respond electrochemically to the catalytic activity of the enzyme in the presence of a suitable analyte.
Electrochemical biosensors are well known in the art. They are used in measurement techniques including amperometry, coulometry and potentiometry. The biosensor comprises a working electrode and a counter electrode to complete an electric circuit. A reference electrode may also be used, to help maintain a constant potential between the working and counter electrodes. The reference and counter electrodes may be combined as a reference/counter electrode.
Typically the enzyme is an oxidoreductase, for example glucose oxidase, cholesterol oxidase, or lactate oxidase, which produces hydrogen peroxide according to the reaction:analyte+O2−[oxidase]→oxidised product+H2O2.
In an amperometric measurement, the peroxide is oxidised at a fixed-potential working electrode as follows:H2O2→O2+2H++2e−.
Electrochemical oxidation of hydrogen peroxide at platinum centres on the working electrode results in transfer of electrons from the peroxide to the electrode producing a current which is proportional to the analyte concentration. Where glucose is the analyte, the oxidised product is gluconolactone.
In coulometric measurement, the current passed during completion or near completion of electrolysis of the analyte is measured and integrated to give a value of charge passed. The charge passed is related to the quantity of analyte present in a sample so that if the sample volume is known the analyte concentration can be determined. In potentiometric measurement, a potential generated by the reaction is measured at one or more points in time and related to the initial analyte concentration. The various electrochemical measurement techniques are well known to those skilled in the art.
Typically, electrochemical measurement begins automatically when the fluid sample completes an electrical circuit between the working and counter electrodes. Getting an accurate reading can be a problem when a blood sample incompletely covers the working electrode because the amount of current or measured charge is less than when the working electrode is fully covered. If a user attempts to top-up the sample by applying a second drop of blood (‘double-dosing’) this has the effect of reducing the precision of the measurement and increasing the response as the addition of extra blood causes a non-faradaic charging peak to occur when more of the electrode area is covered by the second sample.
It has been proposed to reduce the problem of incomplete fill by employing a pair of fill-detection electrodes in the fluid path, with the working and counter electrodes inbetween. A measurement is only taken when a circuit has been completed between the fill electrodes. However, this arrangement adds complexity to the system and does not address the problems of double-dosing by the user.